Method of forming electrostatic latent images

ABSTRACT

In a method of forming an electrostatic latent image by the steps of applying a first electric field across a photosensitive element including a photosensitive layer and a highly insulative layer so as to deposit a first uniform change of one polarity upon the surface of the highly insulative layer and to establish a uniform charge distribution in the photosensitive layer, applying a second electric field across the photosensitive element to deposit a second charge of the opposite polarity and projecting a light image upon the photosensitive element concurrently with the application of the second field, the second field is applied in two spaced-apart periods and the light image is projected between the periods.

United States Patent METHOD OF FORMING ELECTROSTATIC LATENT IMAGES 6Claims, 1 Drawing Fig.

u.s. Cl ass/17,

ass/3, 96/1, 346/74 ES 1111.01 603; 13/22 Field ofSeareh 355/17, 3

[ 56] References Cited UNITED STATES PATENTS 3,041,167 6/1962 Blakney etal. 355/1 7 3,307,034 2/1967 Bean 355/3 Primary Examiner-Samuel S.Matthews Assistant Examiner-E. M. Bero Attorney-Bosworth, Sessions,Herrstrom and Cain ABSTRACT: In a method of forming an electrostaticlatent image by the steps of applying a first electric field acroas aphotosensitive element including a photosensitive layer and a highlyinsulative layer so as to deposit a first uniform change of one polarityupon the surface of the highly insulative layer and to establish auniform charge distribution in the photosensitive layer, applying asecond electric field across the photosensitive element to deposit asecond charge of the op posite polarity and projecting a light imageupon the photosensitive element concurrently with the application of thesecond field, the second field is applied in two spacedapart periods andthe light image is projected between the periods.

BACKGROUND OF THE INVENTION This invention relates to a method offorming electrostatic latent images in an electrophotography utilizingcorona discharge electrodes in the form of fine wires and moreparticularly to an improved method of eliminating the effect of coronadischarge electrodes upon the projected light image.

According to a recently developed method of electrophotography describedmore fully, for example in U.S. Pat. No. 3,457,070, issued July 22,l969,and in copending U.S. application, Ser. No. 481,365 filed Aug. 20,1965,.hereinafter referred to as the KTA process, an electrostaticlatent image is formed by the steps of applying a first electric fieldacross a photosensitive element including a thin and transparent highlyinsulative layer and photosensitive layer integrally bonded to thehighly insulative layer to deposit a charge of a first polarity upon thesurface thereof, applying a second electric field across thephotosensitive element to deposit a charge of opposite polarity, andprojecting a light image upon the photosensitive element concurrentlywith the application of the second field whereby to form anelectrostatic latent image on the surface of the highly insulative layercorresponding to the projected light image.

Where a corona discharge electrode in the form of fine metal wires isused to establish the first and second fields, as the light image isprojected through the metal wires while the second field is beingapplied across the photosensitive element the shadow of these metalwires afiects the quality of the electrostatic latent image or a visibleimage reproduced therefrom. in order to eliminate this problem it hasbeen tried to oscillate or shift the corona discharge device. However,this approach was not satisfactory because the interval during which aphotographic flash lamp flashes is very short.

SUMMARY OF THE INVENTION It is therefore the principal object of thisinvention to provide a novel method of forming electrostatic latentimages which are free from the shadow of corona discharge metal wires.

Briefly stated, according to one embodiment of this invention, themethod offorming electrostatic latent images comprises applying a firstelectric field across a photosensitive element including aphotosensitive layer manifesting persistent internal polarization and ahighly insulative layer integrally bonded to the photosensitive layer soas to deposit a first uniform charge upon the surface of the highlyinsulative layer and to establish a uniform charge polarization in thephotosensitive layer; applying a second field across the photosensitiveelement in the dark to deposit on the surface of the highly insulativelayer a second charge of the opposite polarity; projecting a light imageupon the photosensitive element; depositing a charge of the oppositepolarity on the surface of the highly insulative layer in the darkwhereby to form electrostatic latent image on the surface of the highlyinsulativc layer; and exposing the surface of the photosensitive elementto uniform light.

The latent images produced in this manner can be developed and transferprinted in the well-known manner.

Where the light image is projected upon the photosensitive elementconcurrently with the application of the second field of the oppositepolarity by means of a corona discharge electrodeunitasintheKTAprocess,sincetherateofchargingthe surface of the photosensitiveelement by the action of corona discharge is not high, the field appliedacross the photosensitive layer does not increase rapidly. Thus, duringsuch interval wherein the field intensity is still low, it is impossibleto expect high photosensitivity. For this reason, with the KTA process,it is impossible to obtain high photoeensitivity unless the time oflight image projection is carefully preselected. The novel method cancompletely solve this problem.

The photosensitive layer to be used in this invention '3 desirable tohave a high dark resistance. However, it is more desirable to use aphotosensitive layer which includes a plurality of trap levels in thelayer at portions near the highly insulative layer integrally bonded tothe photosensitive layer, or a photosensitive layer that can manifestpersistent internal polarization (PIP) effect. With such aphotosensitive layer it is possible to increase the interval between thesecond and fourth steps without decreasing the intensity of theelectrostatic latent image.

BRIEF DESCRIPTION OF THE DRAWING This invention can be more fullyunderstood from the following detailed description taken in connectionwith the accompanying drawing in which a single FIGURE diagrammaticallyshows an electrophotographic apparatus utilized to carry out thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1.

Vapor of monomer of acrylic resin was introduced into an evacuatedvessel and a high AC voltage was impressed across two spaced-apartelectrode plates disposed in the evacuated vessel to deposit films ofthe polymer of the acrylic resin on the surfaces of the electrode platesto a thickness of about 3 microns. The coated electrode plate wasutilized as a substrate and an alloy of SeTe containing 25 percent of Tewas vapor deposited on the polymer filrn to a thickness of about 34microns. Near the end of the vapor deposition process, Se was also vapordeposited together with the SeTe alloy and thereafter a thin layer ofSeTe along was deposited to obtain a photosensitive layer. A layer ofpolycarbonate was then ap- Further, the photosensitive element can beused repeatedly by erasing the latent image and cleaning the surface ofthe ele--* ment. The novel process can assure extremely highphotosensitivity comparable with that of the KTA process.

The novel method is not only suitable for forming latent images byprojecting the light image through the corona discharge electrode unitbut also can form more stable latent images than the KTA process by thefollowing reason. Thus, in the second step, a large field is appliedacross the photosensitive layer in the dark and then the light image isprojected in the third step so that the density of charge carriersflowing through the photosensitive layer in response to light increasesgreatly thus assuring high photosensitivity.

plied on the photosensitive layer to a thickness of about 15 microns tocomplete a photosensitive element.

A photosensitive element 10 was wrapped around a metal or glass cylinder11 with its highly insulative layer 12 faced outwardly. Thephotosensitive element 10 shown in the drawing further comprised aphotosensitive layer 1.3 prepared in the above-described manner and abacking electrode 14 which is grounded as shown. in some applicationsthe backing electrode 14 is transparent. The cylinder was rotated in thedirection indicated by an arrow.

A first corona discharge electrode unit 15 was placed close to theperiphery of the photosensitive element to deposit a charge of -800volts on the surface of highly insulative layer 12. Then, in the dark, apositive charge was deposited by means of a second corona dischargeelectrode unit 16 until the potential of the surface of the highlyinsulative layer increased to substantially zero volt. 0.5 second aftertermination of this positive corona discharge a suitable light image wasprojected upon the photosensitive element by means of a flash lightoptical system 17, and 0.5 second thereafter, the surface of thephotosensitive element was charged positively by means of a thirdpositive corona discharge electrode 18. Then uniform light, which may beroom light, was projected upon the photosensitive element asschematically shown by arrow A.

Under these conditions, the surface potential was measured and obtaineda charge potential of +200 volts at portions of the highly insulatinglayer corresponding to bright portions of the light image. Theelectrostatic latent image was developed by applying a powder of chargeddeveloper by means of a magnetic bnrsh 19 which are commonly used in theelectrophotography to obtain an intense visible image. The developedpower imagewas'then transfer printed onto a paper 20 in the conventionalmanner. The developer powder remaining on the photosensitive elementafter transfer printing was removed by a cleaning brush 21. Theabove-described cycle of operation was repeated many times without anytrouble.

EXAMPLE 2.

A photosensitive element was prepared comprising an electrode, aphotosensitive layer including a photoconductive layer and a charge traplayer, and a highly insulative layer which were bonded together into anintegral structure. The electrode may be made of a metal plate, lowresistance paper, low resistance synthetic resin, Nesa (trade mark)glass or any other low resistance material. The photoconductive layermay be sintered CdS or CdSe, or vapor deposited CdS, CdSe or Se or athin layer of a powder of CdS, CdSe or ZnO bonded by a binder of a verylow proportion or a thin layer of polyvinyl carbazole whereas the chargetrap layer may be composed of ZnS or ZnCd activated with Cu, Ag or Pb,or anthracene, anthraguinone, S, PbO or the like having a large numberof impurity levels. The highly insulative layer may be made of anymaterial provided that it can transmit light rays and has highinsulating strength. Where Nesa glass is utilized as the electrode, thehighly insulative layer may be opaque.

The method of this invention described in connection with example 1 wascarried out with various photosensitive elements described above andobtained electrostatic latent images similar to that of example 1.

EXAMPLE 3.

The trap layer was formed by diffusing an impurity into the surface of aphotoconductive layer at a high density, thus forming a charge traplayer in the surface portion of the photoconductive layer. Thus, forexample, a charge trap layer was fonned in the surface layer portion ofa sintered photoconductive layer by difi'using an impurity of highconcentration into a surface layer portion at a relatively lowtemperature for a short interval. Other layers were prepared in the samemanner as in example 2.

It was found that this photosensitive element also showed satisfactoryresults.

Stated in another way, this invention comprises a modification of theKTA process wherein the step of applying the second field of the KTAprocess is divided into two spacedapart independent periods and betweenthese periods is interposed the projection of the light imageindependently of the corona discharge, thus eliminating the deleteriouseffect of the shadow of the corona discharge electrode upon the latentimage.

Where photosensitive materials having high PiP effect or high darkresistance are utilized, the method of this invention may be modified byapplying such extremely high potential across the photosensitive elementduring the second step that is not suitable for satisfactory developmentof the latent image and increases the density of drifting chargecarriers contributing to image formation at the time of light imageprojection and by readjusting the surface potential to a value suitablefor developing during the fourth step. This modified method can greatlyimprove the photosensitivity. The field applied in the fourth step maybe a DC or an AC field.

I claim:

1. In a method of forming an electrostatic latent image by the steps ofapplying a first electric field across a photosensitive elementincluding a photosensitive layer manifesting persistent internalpolarization and a highly insulative layer integrally bonded to thephotosensitive layer so as to deposit a first uniform charge of onepolarity upon the surface of the highly insulative layer and toestablish a uniform charge po arrzatron In the photosensitive layer,applying a second electric field across the photosensitive element todeposit on the surface of the highly insulative layer a second charge ofthe opposite polarity, and projecting a light image upon thephotosensitive element, the improvement which comprises applying saidsecond field in two spaced-apart periods, and projecting said lightimage between said periods.

2. The method of forming an electrostatic latent image according toclaim 1 wherein said first and second fields are applied by coronadischarge electrode units.

3. The method of forming an electrostatic latent image according toclaim 1 wherein said electrostatic latent image formed on the surface ofsaid highly insulated layer is subjected to light irradiation anddeveloped under ambient light.

4. An electrophotographic apparatus comprising a photosensitive elementincluding a photosensitive layer manifesting persistent internalpolarization and a highly insulative layer integrally bonded to saidphotosensitive layer, a first corona discharge electrode unit to deposita charge of first polarity on the surface of said highly insulativelayer, spaced-apart second and third corona discharge electrode units todeposit a charge of the opposite polarity on the surface of said highlyinsulative layer, means interposed between said second and third coronadischarge electrode units to project a light image on saidphotosensitive element, whereby to form an electrostatic latent image onthe surface of said highly insulative layer corresponding to saidprojected light image, means to develop said latent image as a powderimage, means to transfer print said powder image onto a receptivemedium, means to clean the surface of said photosensitive element andmeans for moving said photosensitive element relative to other elementsof the apparatus.

5. The electrophotographic apparatus according to claim 4 wherein saidapparatus further comprises means interposed between said third coronadischarge electrode unit and said developing means to irradiate saidphotosensitive element with uniform light.

6. The electrophotographic apparatus according to claim 4 wherein saidphotosensitive element is wrapped around a rotary cylinder with saidhighly insulative layer faced outwardly, and said first to third coronadischarge electrode units. light image projecting means, said developingmeans, transfer means and cleaning means are disposed around theperiphery of said photosensitive element.

2. The method of forming an electrostatic latent image according toclaim 1 wherein said first and second fields are applied by coronadischarge electrode units.
 3. The method of forming an electrostaticlatent image according to claim 1 wherein said electrostatic latentimage formed on the surface of said highly insulated layer is subjectedto light irradiation and developed under ambient light.
 4. Anelectrophotographic apparatus comprising a photosensitive elementincluding a photosensitive layer manifesting persistent internalpolarization and a highly insulative layer integrally bonded to saidphotosensitive layer, a first corona discharge electrode unit to deposita charge of first polarity on the surface of said highly insulativelayer, spaced-apart second and third corona discharge electrode units todeposit a charge of the opposite polarity on the surface of said highlyinsulative layer, means interposed between said second and third coronadischarge electrode units to project a light image on saidphotosensitive element, whereby to form an electrostatic latent image onthe surface of said highly insulative layer corresponding to saidprojected light image, means to develop said latent image as a powderimage, means to transfer print said powder image onto a receptivemedium, means to clean the surface of said photosensitive element andmeans for moving said photosensitive element relative to other elementsof the apparatus.
 5. The electrophotographic apparatus according toclaim 4 wherein said apparatus further comprises means interposedbetween said third corona discharge electrode unit and said developingmeans to irradiate said photosensitive element with uniform light. 6.The electrophotographic apparatus according to claim 4 wherein saidphotosensitive element is wrapped around a rotary cylinder with saidhighly insulative layer faced outwardly, and said first to third coronadischarge electrode units, light image projecting means, said developingmeans, transfer means and cleaning means are disposed around theperiphery of said photosensitive element.